SOURCE

<div class="box t1">
    <div class="box t3"></div>
</div>
<div class="box t2">
    <div class="box t4"></div>
    <div class="box t5"></div>
</div>

.box {
    width: 120px;
    height: 120px;
}
.t1 {
    position: relative;
    background: red;
    top: 30px;
    left: 0;
}
.t3 {
    position: absolute;
    top: 100px;
    z-index: 100011;
    background: blue;
}
.t2 {
    z-index: 2;
    position: absolute;
    background: green;
    top: 120px;
    left: 0;
}
.t4 {
    position: absolute;
    top: 80px;
    z-index: 10000;
    background: orange;
}
.t5 {
    left: 40px;
    background-color: pink;
}